Basics To Antenna Theory

Overview At low frequencies, an electrical circuit is completely characterized by the electrical parameters like resistance, inductance etc. and the physical size of the electrical components plays no role in the circuit analysis. As the frequency increases however, the size of the components becomes important, that is to say that, the space starts playing a role in the performance of the circuit. The voltage and currents exist in the form of waves. Even a change in the length of a simple connecting wire may alter the behavior of the circuit. The circuit approach then has to be re-investigated with inclusion of the space into the analysis. This approach is then called the transmission line approach.

One can then conveniently divide the subject of electromagnetics in two parts, the static electromagnetics and the time varying electromagnetics. As will be clear subsequently, the time varying electric and magnetic fields always constitute a wave phenomenon called the electromagnetic wave which is the prime subject of discussion of this book. The phenomenon of electromagnetism in totality is governed by the four Maxwell's equations, which can be derived from the physical laws like the Gauss Law, the Ampere's law and the Faraday's low of electromagnetic induction. The electromagnetic theory is the generalization of the circuit theory, or the circuit theory is rather a special case of the electromagnetic theory. Although every phenomena of electricity and magnetism can be analyzed in the frame work of electromagnetic theory, at low frequencies the circuit approach is adequate. As the frequency increases the inadequacy of the circuit approach is felt and one is forced to follow the electromagnetic field approach. Objectives

At low frequencies, an electrical circuit is completely characterized by the electrical parameters like resistance, inductance etc. and the physical size of the electrical components plays no role in the circuit analysis. As the frequency increases however, the size of the components becomes important, that is to say that, the space starts playing a role in the performance of the circuit. The voltage and currents exist in the form of waves. Even a change in the length of a simple connecting wire may alter the behavior of the circuit. The circuit approach then has to be re-

investigated with inclusion of the space into the analysis. This approach is then called the transmission line approach.

The voltage and currents exist in the form of waves. Even a change in the length of a simple connecting wire may alter the behavior of the circuit. The circuit approach then has to be re-investigated with inclusion of the space into the analysis. This approach is then called the transmission line approach. Although the primary objective of a transmission line is to carry electromagnetic energy efficiently from one location to other, they find wide applications in high frequency circuit design. As

the frequency increases, any discontinuity in the circuit path leads to electromagnetic radiation. Also at high frequencies, the transit time of the signals can not be ignored. In the era of high speed computers, where data rates are approaching to few Gb/sec, the phenomena related to the electromagnetic waves, like the bit distortion, signal reflection, impedance matching play a vital role in high speed communication networks. An antenna is a device which can launch and receive electromagnetic waves efficiently. But for the large antennas, the communication between an earth station and a satellite is practically impossible. The communication which can be established with few watts of power, would need few MW of power in the absence of proper antennas. However, antenna research is still very active. With recent advances in mobile communication, design of compact, efficient, multi-frequency antennas have received a new impetus in the last decade.